JavaScript has several variables

Javascript has three types of variables, namely: 1. Global variables, which are declared outside all functions; 3. Local variables, which are variables declared within the function body or named parameters of the function; 3. , Block-level variables, which are variables declared in the block, are only valid in the block.

The operating environment of this tutorial: windows7 system, javascript version 1.8.5, Dell G3 computer.

Javascript has three types of variables, namely global variables, local variables and block-level variables.

Global variables are declared outside all functions; local variables are variables declared in the function body or named parameters of the function; block-level variables are variables declared in the block and are only valid in the block.

Three types of variables correspond to three types of scope: global scope, local scope (also called function scope), and block-level scope; the scope of a variable is closely related to the declaration method.

The scope of variables declared using var has global scope and function scope, and there is no block-level scope; variables declared using let and const have global scope, local scope and block-level scope.

Note: Global variables in the strict sense belong to the properties of the window object, but variables declared by let and const do not belong to the window object, so they are not global variables in the strict sense. Here they are just They are global variables from the perspective of their scope.

Since var supports variable promotion, the global scope of the var variable is valid for the script code of the entire page; while let and const do not support variable promotion, so the global scope of the let and const variables refers to is the entire area from the beginning of the declaration statement to the end of the script code of the entire page, and the area before the declaration statement is invalid.

Similarly, because var supports variable promotion, but let and const do not support variable promotion, local variables declared using var are valid throughout the function, while local variables declared using let and const are valid from the beginning of the declaration statement to The area between the end of the function is valid. It should be noted that if the local variable and the global variable have the same name, in the function scope, the local variable will overwrite the global variable, that is, the local variable will work in the function body; outside the function body, the global variable will work, and the local variable will work. The variable is invalid, and a syntax error will occur when referencing local variables.

For block-level variables, their scope is the area between the beginning of the block-level variable declaration statement and the end of the block. The area between the beginning of the block and the block-level variable declaration statement is the "temporary dead zone". In this area, the block-level variables are not valid.

In addition, in non-strict running mode, variables do not need to be declared. These undeclared variables are global variables no matter where they are used. It is generally not recommended to use variables directly without declaring them, because this may cause some errors that are not easy to find.

[Recommended learning: javascript advanced tutorial]

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